Although the exact cause of breast cancer is unknown as of March 2015, the condition always begins with the abnormal growth of breast cells, according to Mayo Clinic. A number of hormonal, environmental and lifestyle risk factors may contribute to the illness.
Certain gene mutations, such as those of BRCA1 and BRCA2, drastically increase a person's chance of developing breast cancer and can be passed down through generations, according to the American Cancer Society. The majority of breast cancer cases are not caused by this type of inherited DNA mutation, instead resulting from acquired mutations that appear over a person's lifetime. Most cases result from more than one acquired mutation, and many of the causes are not yet known. Radiation and certain carcinogenic chemicals have been definitively linked to breast cancer.
Individuals have an increased risk of developing breast cancer if they are female, consume alcohol, are obese or are over age 35 when they have their first child, states Mayo Clinic. Starting menstruation before age 12, beginning menopause at an older age, undergoing postmenopausal therapy and never having children also put women at a greater risk. The chances of developing breast cancer increase with age, and those with a family history of breast cancer are more likely to be diagnosed with the condition.